igraph_error_t igraph_read_graph_edgelist(igraph_t *graph, FILE *instream,
                               igraph_integer_t n, igraph_bool_t directed);

This format is simply a series of an even number of non-negative integers separated by whitespace. The integers represent vertex IDs. Placing each edge (i.e. pair of integers) on a separate line is not required, but it is recommended for readability. Edges of directed graphs are assumed to be in "from, to" order.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|), the number of vertices plus the number of edges. It is assumed that reading an integer requires O(1) time.

igraph_error_t igraph_write_graph_edgelist(const igraph_t *graph, FILE *outstream);

Edges are represented as pairs of 0-based vertex indices. One edge is written per line, separated by a single space. For directed graphs edges are written in from, to order.

Arguments: 

Returns: 

Time complexity: O(|E|), the number of edges in the graph. It is assumed that writing an integer to the file requires O(1) time.

igraph_error_t igraph_read_graph_ncol(igraph_t *graph, FILE *instream,
                           const igraph_strvector_t *predefnames,
                           igraph_bool_t names,
                           igraph_add_weights_t weights,
                           igraph_bool_t directed);

Also useful for creating graphs from “named” (and optionally weighted) edge lists.

This format is used by the Large Graph Layout program (http://lgl.sourceforge.net), and it is simply a symbolic weighted edge list. It is a simple text file with one edge per line. An edge is defined by two symbolic vertex names separated by whitespace. The vertex names themselves cannot contain whitespace. They may be followed by an optional number, the weight of the edge; the number can be negative and can be in scientific notation. If there is no weight specified to an edge it is assumed to be zero.

The resulting graph is always undirected. LGL cannot deal with files which contain multiple or loop edges, this is however not checked here, as igraph is happy with these.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|log(|V|)) if we neglect the time required by the parsing. As usual |V| is the number of vertices, while |E| is the number of edges.

See also: 

igraph_error_t igraph_write_graph_ncol(const igraph_t *graph, FILE *outstream,
                            const char *names, const char *weights);

.ncol is a format used by LGL, see igraph_read_graph_ncol() for details.

Note that having multiple or loop edges in an .ncol file breaks the LGL software but igraph does not check for this condition.

This format cannot represent zero-degree vertices.

Arguments: 

Returns: 

Time complexity: O(|E|), the number of edges. All file operations are expected to have time complexity O(1).

See also: 

igraph_error_t igraph_read_graph_lgl(igraph_t *graph, FILE *instream,
                          igraph_bool_t names,
                          igraph_add_weights_t weights,
                          igraph_bool_t directed);

The .lgl format is used by the Large Graph Layout visualization software (http://lgl.sourceforge.net), it can describe undirected optionally weighted graphs. From the LGL manual:

LGL cannot handle loop and multiple edges or directed graphs, but in igraph it is not an error to have multiple and loop edges.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|log(|V|)) if we neglect the time required by the parsing. As usual |V| is the number of vertices, while |E| is the number of edges.

See also: 

#include <igraph.h>

int main(void) {

    igraph_t g;
    FILE *input;

    /* Turn on attribute handling. */
    igraph_set_attribute_table(&igraph_cattribute_table);

    /* Without names and weights */
    input = fopen("igraph_read_graph_lgl-1.lgl", "r");
    if (!input) {
        return 1;
    }
    igraph_read_graph_lgl(&g, input, 0, IGRAPH_ADD_WEIGHTS_NO, 1);
    fclose(input);
    if (!igraph_is_directed(&g)) {
        return 2;
    }
    igraph_write_graph_edgelist(&g, stdout);
    igraph_destroy(&g);

    /* With names and weights */
    input = fopen("igraph_read_graph_lgl-2.lgl", "r");
    if (!input) {
        return 3;
    }
    igraph_read_graph_lgl(&g, input, 0, IGRAPH_ADD_WEIGHTS_NO, 1);
    fclose(input);
    if (!igraph_is_directed(&g)) {
        return 4;
    }
    igraph_write_graph_ncol(&g, stdout, 0, 0);
    igraph_destroy(&g);

    /* Same graph, but forcing undirected mode */
    input = fopen("igraph_read_graph_lgl-2.lgl", "r");
    igraph_read_graph_lgl(&g, input, 0, IGRAPH_ADD_WEIGHTS_NO, 0);
    fclose(input);
    if (igraph_is_directed(&g)) {
        return 5;
    }
    igraph_write_graph_ncol(&g, stdout, 0, 0);
    igraph_destroy(&g);

    /* Erroneous LGL file (empty vertex name) */
    input = fopen("igraph_read_graph_lgl-3.lgl", "r");
    if (!input) {
        return 6;
    }
    igraph_set_error_handler(igraph_error_handler_ignore);
    if (igraph_read_graph_lgl(&g, input, 0, IGRAPH_ADD_WEIGHTS_NO, 1) !=
        IGRAPH_PARSEERROR) {
        return 7;
    }
    fclose(input);

    return 0;
}

igraph_error_t igraph_write_graph_lgl(const igraph_t *graph, FILE *outstream,
                           const char *names, const char *weights,
                           igraph_bool_t isolates);

.lgl is a format used by LGL, see igraph_read_graph_lgl() for details.

Note that having multiple or loop edges in an .lgl file breaks the LGL software but igraph does not check for this condition.

Arguments: 

Returns: 

Time complexity: O(|E|), the number of edges if isolates is false, O(|V|+|E|) otherwise. All file operations are expected to have time complexity O(1).

See also: 

#include <igraph.h>

int main(void) {

    igraph_t graph;
    igraph_strvector_t names;
    igraph_vector_t weights;
    igraph_integer_t i;
    igraph_integer_t vcount, ecount;

    igraph_set_attribute_table(&igraph_cattribute_table);

    igraph_small(&graph, 7, IGRAPH_UNDIRECTED,
                 0,1, 1,3, 1,2, 2,0, 4,2, 3,4,
                 -1);
    vcount = igraph_vcount(&graph);
    ecount = igraph_ecount(&graph);


    printf("Output without isolates:\n");
    igraph_write_graph_lgl(&graph, stdout, /*names*/ NULL, /*weights*/ NULL, /*isolates*/ 0);


    printf("\nOutput with isolates:\n");
    igraph_write_graph_lgl(&graph, stdout, /*names*/ NULL, /*weights*/ NULL, /*isolates*/ 1);


    printf("\nOutput vertex and edge labels:\n");
    igraph_strvector_init(&names, vcount);
    for (i = 0; i < vcount; i++) {
        char str[2] = " "; /* initialize to ensure presence of null terminator */
        str[0] = 'A' + i;
        igraph_strvector_set(&names, i, str);
    }
    SETVASV(&graph, "names", &names);

    igraph_vector_init_range(&weights, 1, ecount + 1);
    SETEANV(&graph, "weights", &weights);

    igraph_write_graph_lgl(&graph, stdout, "names", "weights", /*isolates*/ 0);

    igraph_strvector_destroy(&names);
    igraph_vector_destroy(&weights);
    igraph_destroy(&graph);

    return 0;
}

igraph_error_t igraph_read_graph_dimacs_flow(
        igraph_t *graph, FILE *instream,
        igraph_strvector_t *problem,
        igraph_vector_int_t *label,
        igraph_integer_t *source,
        igraph_integer_t *target,
        igraph_vector_t *capacity,
        igraph_bool_t directed);

This function reads the DIMACS file format, more specifically the version for network flow problems, see the files at http://archive.dimacs.rutgers.edu/pub/netflow/general-info/

This is a line-oriented text file (ASCII) format. The first character of each line defines the type of the line. If the first character is c the line is a comment line and it is ignored. There is one problem line (p in the file), it must appear before any node and arc descriptor lines. The problem line has three fields separated by spaces: the problem type (max or edge), the number of vertices, and number of edges in the graph. In MAX problems, exactly two node identification lines are expected (n), one for the source, and one for the target vertex. These have two fields: the ID of the vertex and the type of the vertex, either s ( = source) or t ( = target). Arc lines start with a and have three fields: the source vertex, the target vertex and the edge capacity. In EDGE problems, there may be a node line (n) for each node. It specifies the node index and an integer node label. Nodes for which no explicit label was specified will use their index as label. In EDGE problems, each edge is specified as an edge line (e).

Within DIMACS files, vertex IDs are numbered from 1.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|+c), the number of vertices plus the number of edges, plus the size of the file in characters.

See also: 

igraph_error_t igraph_write_graph_dimacs_flow(const igraph_t *graph, FILE *outstream,
                              igraph_integer_t source, igraph_integer_t target,
                              const igraph_vector_t *capacity);

This function writes a graph to an output stream in DIMACS format, describing a maximum flow problem. See ftp://dimacs.rutgers.edu/pub/netflow/general-info/

This file format is discussed in the documentation of igraph_read_graph_dimacs_flow(), see that for more information.

Arguments: 

Returns: 

Time complexity: O(|E|), the number of edges in the graph.

See also: 

igraph_error_t igraph_read_graph_graphdb(igraph_t *graph, FILE *instream,
                              igraph_bool_t directed);

This is a binary format, used in the ARG Graph Database for isomorphism testing. For more information, see https://mivia.unisa.it/datasets/graph-database/arg-database/

From the graph database homepage:

As of igraph 0.10, only unlabelled graphs are implemented.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|), the number of vertices plus the number of edges.

#include <igraph.h>

int main(void) {

    igraph_t g;
    FILE *input;

    input = fopen("iso_b03_m1000.A00", "rb");
    if (!input) {
        return 1;
    }
    igraph_read_graph_graphdb(&g, input, IGRAPH_DIRECTED);
    fclose(input);
    igraph_write_graph_edgelist(&g, stdout);
    igraph_destroy(&g);

    return 0;
}

igraph_error_t igraph_read_graph_graphml(igraph_t *graph, FILE *instream, igraph_integer_t index);

GraphML is an XML-based file format for representing various types of graphs. Currently only the most basic import functionality is implemented in igraph: it can read GraphML files without nested graphs and hyperedges. Attributes of the graph are loaded only if an attribute interface is attached, see igraph_set_attribute_table(). String attrribute values are returned in UTF-8 encoding.

Graph attribute names are taken from the attr.name attributes of the key tags in the GraphML file. Since attr.name is not mandatory, igraph will fall back to the id attribute of the key tag if attr.name is missing.

Arguments: 

Returns: 

#include <igraph.h>
#include <stdio.h>
#include <unistd.h> /* unlink */

int main(void) {
    igraph_t graph;
    const char *infilename  = "test.graphml";
    const char *outfilename = "test2.graphml";

    /* Set up attribute handling, so graph attributes can be imported
     * from the GraphML file. */
    igraph_set_attribute_table(&igraph_cattribute_table);

    /* Problems in the GraphML file may cause igraph to print warnings.
     * If this is not desired, set a silent warning handler: */
    igraph_set_warning_handler(&igraph_warning_handler_ignore);

    /* Read the contents of a GraphML file. */

    /* GraphML */
    FILE *infile = fopen("test.graphml", "r");
    if (! infile) {
        fprintf(stderr, "Could not open input file '%s'.", infilename);
        exit(1);
    }

    /* GraphML support is an optional feature in igraph. If igraph was compiled
     * without GraphML support, igraph_read_graph_graphml() returns IGRAPH_UNIMPLEMENTED.
     * We temporarily disable the default error handler so we can test for this condition. */
    igraph_error_handler_t *oldhandler = igraph_set_error_handler(igraph_error_handler_ignore);
    igraph_error_t ret = igraph_read_graph_graphml(&graph, infile, 0);
    if (ret == IGRAPH_UNIMPLEMENTED) {
        fprintf(stderr, "igraph was compiled without GraphML support.");
        exit(77);
    }
    if (ret != IGRAPH_SUCCESS) {
        fprintf(stderr, "Unexpected error while reading GraphML.");
        exit(1);
    }
    igraph_set_error_handler(oldhandler);


    fclose(infile);

    /* Write it back into another file. */

    FILE *outfile = fopen(outfilename, "w");
    if (outfile) {
        igraph_write_graph_graphml(&graph, outfile, true);
        fclose(outfile);

        /* Clean up after ourselves */
        unlink(outfilename);
    } else {
        fprintf(stderr, "Could not write output file '%s'.", outfilename);
    }

    /* Destroy the graph */
    igraph_destroy(&graph);

    return 0;
}

igraph_error_t igraph_write_graph_graphml(const igraph_t *graph, FILE *outstream,
                               igraph_bool_t prefixattr);

GraphML is an XML-based file format for representing various types of graphs. See the GraphML Primer (http://graphml.graphdrawing.org/primer/graphml-primer.html) for detailed format description.

When a numerical attribute value is NaN, it will be omitted from the file.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|) otherwise. All file operations are expected to have time complexity O(1).

#include <igraph.h>
#include <stdio.h>
#include <unistd.h> /* unlink */

int main(void) {
    igraph_t graph;
    const char *infilename  = "test.graphml";
    const char *outfilename = "test2.graphml";

    /* Set up attribute handling, so graph attributes can be imported
     * from the GraphML file. */
    igraph_set_attribute_table(&igraph_cattribute_table);

    /* Problems in the GraphML file may cause igraph to print warnings.
     * If this is not desired, set a silent warning handler: */
    igraph_set_warning_handler(&igraph_warning_handler_ignore);

    /* Read the contents of a GraphML file. */

    /* GraphML */
    FILE *infile = fopen("test.graphml", "r");
    if (! infile) {
        fprintf(stderr, "Could not open input file '%s'.", infilename);
        exit(1);
    }

    /* GraphML support is an optional feature in igraph. If igraph was compiled
     * without GraphML support, igraph_read_graph_graphml() returns IGRAPH_UNIMPLEMENTED.
     * We temporarily disable the default error handler so we can test for this condition. */
    igraph_error_handler_t *oldhandler = igraph_set_error_handler(igraph_error_handler_ignore);
    igraph_error_t ret = igraph_read_graph_graphml(&graph, infile, 0);
    if (ret == IGRAPH_UNIMPLEMENTED) {
        fprintf(stderr, "igraph was compiled without GraphML support.");
        exit(77);
    }
    if (ret != IGRAPH_SUCCESS) {
        fprintf(stderr, "Unexpected error while reading GraphML.");
        exit(1);
    }
    igraph_set_error_handler(oldhandler);


    fclose(infile);

    /* Write it back into another file. */

    FILE *outfile = fopen(outfilename, "w");
    if (outfile) {
        igraph_write_graph_graphml(&graph, outfile, true);
        fclose(outfile);

        /* Clean up after ourselves */
        unlink(outfilename);
    } else {
        fprintf(stderr, "Could not write output file '%s'.", outfilename);
    }

    /* Destroy the graph */
    igraph_destroy(&graph);

    return 0;
}

igraph_error_t igraph_read_graph_gml(igraph_t *graph, FILE *instream);

GML is a simple textual format, see https://web.archive.org/web/20190207140002/http://www.fim.uni-passau.de/index.php?id=17297%26L=1 for details.

Although all syntactically correct GML can be parsed, we implement only a subset of this format. Some attributes might be ignored. Here is a list of all the differences:

Please contact us if you cannot live with these limitations of the GML parser.

Arguments: 

Returns: 

Time complexity: should be proportional to the length of the file.

See also: 

#include <igraph.h>
#include <stdio.h>

int main(void) {
    igraph_t graph;
    FILE *ifile;

    ifile = fopen("karate.gml", "r");
    if (ifile == 0) {
        return 1;
    }

    igraph_read_graph_gml(&graph, ifile);
    fclose(ifile);

    printf("The graph is %s.\n", igraph_is_directed(&graph) ? "directed" : "undirected");

    /* Output as edge list */
    printf("\n-----------------\n");
    igraph_write_graph_edgelist(&graph, stdout);

    /* Output as GML */
    printf("\n-----------------\n");
    igraph_write_graph_gml(&graph, stdout,IGRAPH_WRITE_GML_DEFAULT_SW,  0, "");

    igraph_destroy(&graph);

    return 0;
}

igraph_error_t igraph_write_graph_gml(const igraph_t *graph, FILE *outstream,
                                      igraph_write_gml_sw_t options,
                                      const igraph_vector_t *id, const char *creator);

GML is a quite general textual format, see https://web.archive.org/web/20190207140002/http://www.fim.uni-passau.de/index.php?id=17297%26L=1 for details.

The graph, vertex and edges attributes are written to the file as well, if they are numeric or string. Boolean attributes are converted to numeric, with 0 and 1 used for false and true, respectively. NaN values of numeric attributes are skipped, as NaN is not part of the GML specification and other software may not be able to read files containing them. This is consistent with igraph_read_graph_gml(), which produces NaN when an attribute value is missing. In contrast with NaN, infinite values are retained. Ensure that none of the numeric attributes values are infinite to produce a conformant GML file that can be read by other software.

As igraph is more forgiving about attribute names, it might be necessary to simplify the them before writing to the GML file. This way we'll have a syntactically correct GML file. The following simple procedure is performed on each attribute name: first the alphanumeric characters are extracted, the others are ignored. Then if the first character is not a letter then the attribute name is prefixed with “igraph”. Note that this might result identical names for two attributes, igraph does not check this.

The “id” vertex attribute is treated specially. If the id argument is not NULL then it should be a numeric vector with the vertex IDs and the “id” vertex attribute is ignored (if there is one). If id is NULL and there is a numeric “id” vertex attribute, it will be used instead. If ids are not specified in either way then the regular igraph vertex IDs are used. If some of the supplied id values are invalid (non-integer or NaN), all supplied id are ignored and igraph vertex IDs are used instead.

Note that whichever way vertex IDs are specified, their uniqueness is not checked.

If the graph has edge attributes that become “source” or “target” after encoding, or the graph has an attribute that becomes “directed”, they will be ignored with a warning. GML uses these attributes to specify the edge endpoints, and the graph directedness, so we cannot write them to the file. Rename them before calling this function if you want to preserve them.

Arguments: 

Returns: 

Time complexity: should be proportional to the number of characters written to the file.

See also: 

#include <igraph.h>
#include <stdio.h>

int main(void) {
    igraph_t graph;
    FILE *ifile;

    ifile = fopen("karate.gml", "r");
    if (ifile == 0) {
        return 1;
    }

    igraph_read_graph_gml(&graph, ifile);
    fclose(ifile);

    printf("The graph is %s.\n", igraph_is_directed(&graph) ? "directed" : "undirected");

    /* Output as edge list */
    printf("\n-----------------\n");
    igraph_write_graph_edgelist(&graph, stdout);

    /* Output as GML */
    printf("\n-----------------\n");
    igraph_write_graph_gml(&graph, stdout,IGRAPH_WRITE_GML_DEFAULT_SW,  0, "");

    igraph_destroy(&graph);

    return 0;
}

igraph_error_t igraph_read_graph_pajek(igraph_t *graph, FILE *instream);

Arguments: 

Returns: 

Only a subset of the Pajek format is implemented. This is partially because this format is not very well documented, but also because igraph does not support some Pajek features, like multigraphs.

Starting from version 0.6.1 igraph reads bipartite (two-mode) graphs from Pajek files and add the type vertex attribute for them. Warnings are given for invalid edges, i.e. edges connecting vertices of the same type.

The list of the current limitations:

If there are attribute handlers installed, igraph also reads the vertex and edge attributes from the file. Most attributes are renamed to be more informative: color instead of c, xfact instead of x_fact, yfact instead of y_fact, labeldist instead of lr, labeldegree2 instead of lphi, framewidth instead of bw, fontsize instead of fos, rotation instead of phi, radius instead of r, diamondratio instead of q, labeldegree instead of la, vertexsize instead of size, color instead of ic, framecolor instead of bc, labelcolor instead of lc, these belong to vertices.

Edge attributes are also renamed, s to arrowsize, w to edgewidth, h1 to hook1, h2 to hook2, a1 to angle1, a2 to angle2, k1 to velocity1, k2 to velocity2, ap to arrowpos, lp to labelpos, lr to labelangle, lphi to labelangle2, la to labeldegree, fos to fontsize, a to arrowtype, p to linepattern, l to label, lc to labelcolor, c to color.

In addition the following vertex attributes might be added: id if there are vertex IDs in the file, x and y or x and y and z if there are vertex coordinates in the file.

The weight edge attribute might be added if there are edge weights present.

See the pajek homepage: http://vlado.fmf.uni-lj.si/pub/networks/pajek/ for more info on Pajek and the Pajek manual: http://vlado.fmf.uni-lj.si/pub/networks/pajek/doc/pajekman.pdf for information on the Pajek file format.

Time complexity: O(|V|+|E|+|A|), |V| is the number of vertices, |E| the number of edges, |A| the number of attributes (vertex + edge) in the graph if there are attribute handlers installed.

See also: 

#include <igraph.h>
#include <stdio.h>

int main(void) {
    igraph_t g;
    FILE *ifile;

    /* Turn on attribute handling. */
    igraph_set_attribute_table(&igraph_cattribute_table);

    /* Read a Pajek file. */
    ifile = fopen("links.net", "r");
    if (ifile == 0) {
        return 10;
    }
    igraph_read_graph_pajek(&g, ifile);
    fclose(ifile);

    /* Write it in edgelist format. */
    printf("The graph:\n");
    printf("Vertices: %" IGRAPH_PRId "\n", igraph_vcount(&g));
    printf("Edges: %" IGRAPH_PRId "\n", igraph_ecount(&g));
    printf("Directed: %i\n", igraph_is_directed(&g) ? 1 : 0);
    igraph_write_graph_edgelist(&g, stdout);
    igraph_destroy(&g);

    return 0;
}

igraph_error_t igraph_write_graph_pajek(const igraph_t *graph, FILE *outstream);

The Pajek vertex and edge parameters (like color) are determined by the attributes of the vertices and edges, of course this requires an attribute handler to be installed. The names of the corresponding vertex and edge attributes are listed at igraph_read_graph_pajek(), e.g. the color vertex attributes determines the color (c in Pajek) parameter.

As of version 0.6.1 igraph writes bipartite graphs into Pajek files correctly, i.e. they will be also bipartite when read into Pajek. As Pajek is less flexible for bipartite graphs (the numeric IDs of the vertices must be sorted according to vertex type), igraph might need to reorder the vertices when writing a bipartite Pajek file. This effectively means that numeric vertex IDs usually change when a bipartite graph is written to a Pajek file, and then read back into igraph.

Early versions of Pajek supported only Windows-style line endings in Pajek files, but recent versions support both Windows and Unix line endings. igraph therefore uses the platform-native line endings when the input file is opened in text mode, and uses Unix-style line endings when the input file is opened in binary mode. If you are using an old version of Pajek, you are on Unix and you are having problems reading files written by igraph on a Windows machine, convert the line endings manually with a text editor or with unix2dos or iconv from the command line).

Arguments: 

Returns: 

Time complexity: O(|V|+|E|+|A|), |V| is the number of vertices, |E| is the number of edges, |A| the number of attributes (vertex + edge) in the graph if there are attribute handlers installed.

See also: 

#include <igraph.h>

int main(void) {

    igraph_t g;
    igraph_strvector_t names;

    igraph_set_attribute_table(&igraph_cattribute_table);

    /* save a simple ring graph */
    igraph_ring(&g, 10, IGRAPH_DIRECTED, 0 /* mutual */, 1 /* circular */);
    igraph_write_graph_pajek(&g, stdout);

    /* add some vertex attributes */
    igraph_strvector_init(&names, 0);
    igraph_strvector_push_back(&names, "A");
    igraph_strvector_push_back(&names, "B");
    igraph_strvector_push_back(&names, "C");
    igraph_strvector_push_back(&names, "D");
    igraph_strvector_push_back(&names, "E");
    igraph_strvector_push_back(&names, "F");
    igraph_strvector_push_back(&names, "G");
    igraph_strvector_push_back(&names, "H");
    igraph_strvector_push_back(&names, "I");
    igraph_strvector_push_back(&names, "J");
    SETVASV(&g, "id", &names);
    igraph_strvector_destroy(&names);

    /* save the graph with vertex names */
    igraph_write_graph_pajek(&g, stdout);

    igraph_strvector_init(&names, 0);
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "escaping spaces");
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "escaping \\backslashes\\");
    igraph_strvector_push_back(&names, "square");
    igraph_strvector_push_back(&names, "escaping \"quotes\"");
    SETVASV(&g, "shape", &names);
    igraph_strvector_destroy(&names);

    /* save the graph with escaped shapes */
    igraph_write_graph_pajek(&g, stdout);

    /* destroy the graph */
    igraph_destroy(&g);
    return 0;
}

igraph_error_t igraph_read_graph_dl(igraph_t *graph, FILE *instream,
                         igraph_bool_t directed);

This is a simple textual file format used by UCINET. See http://www.analytictech.com/networks/dataentry.htm for examples. All the forms described here are supported by igraph. Vertex names and edge weights are also supported and they are added as attributes. (If an attribute handler is attached.)

Note the specification does not mention whether the format is case sensitive or not. For igraph DL files are case sensitive, i.e. Larry and larry are not the same.

Arguments: 

Returns: 

Time complexity: linear in terms of the number of edges and vertices, except for the matrix format, which is quadratic in the number of vertices.

#include <igraph.h>
#include <stdio.h>
#include <stdlib.h>

int main(void) {

    const char *files[] = { "fullmatrix1.dl", "fullmatrix2.dl",
                            "fullmatrix3.dl", "fullmatrix4.dl",
                            "edgelist1.dl", "edgelist2.dl", "edgelist3.dl",
                            "edgelist4.dl", "edgelist5.dl", "edgelist6.dl",
                            "edgelist7.dl", "nodelist1.dl", "nodelist2.dl" };
    igraph_t graph;
    FILE *infile;

    /* Turn on attribute handling. */
    igraph_set_attribute_table(&igraph_cattribute_table);

    for (size_t i = 0; i < sizeof(files) / sizeof(files[0]); i++) {
        printf("Doing %s\n", files[i]);
        infile = fopen(files[i], "r");
        if (!infile) {
            printf("Cannot open file: %s\n", files[i]);
            abort();
        }
        igraph_read_graph_dl(&graph, infile, IGRAPH_DIRECTED);
        fclose(infile);
        igraph_write_graph_edgelist(&graph, stdout);
        igraph_destroy(&graph);
    }

    if (IGRAPH_FINALLY_STACK_SIZE() != 0) {
        return 1;
    }

    return 0;
}

igraph_error_t igraph_write_graph_dot(const igraph_t *graph, FILE* outstream);

DOT is the format used by the widely known GraphViz software, see http://www.graphviz.org for details. The grammar of the DOT format can be found here: http://www.graphviz.org/doc/info/lang.html

This is only a preliminary implementation, no visualization information is written.

Arguments: 

Time complexity: should be proportional to the number of characters written to the file.

See also: 

#include <igraph.h>
#include <stdio.h>

int main(void) {

    igraph_t g;
    FILE *ifile;

    ifile = fopen("karate.gml", "r");
    if (ifile == 0) {
        return 10;
    }

    igraph_read_graph_gml(&g, ifile);
    fclose(ifile);

    if (igraph_is_directed(&g)) {
        printf("directed\n");
    } else {
        printf("undirected\n");
    }

    igraph_write_graph_edgelist(&g, stdout);
    printf("-----------------\n");
    igraph_write_graph_dot(&g, stdout);
    igraph_destroy(&g);

    return 0;
}

igraph_error_t igraph_write_graph_leda(const igraph_t *graph, FILE *outstream,
                                       const char *vertex_attr_name,
                                       const char *edge_attr_name);

This function writes a graph to an output stream in LEDA format. See http://www.algorithmic-solutions.info/leda_guide/graphs/leda_native_graph_fileformat.html

The support for the LEDA format is very basic at the moment; igraph writes only the LEDA graph section which supports one selected vertex and edge attribute and no layout information or visual attributes.

Arguments: 

Returns: 

Time complexity: O(|V|+|E|), the number of vertices and edges in the graph.

igraph_error_t igraph_enter_safelocale(igraph_safelocale_t *loc);

igraph's foreign format readers and writers require a locale that uses a decimal point instead of a decimal comma. This is a convenience function that temporarily sets the C locale so that readers and writer would work correctly. It must be paired with a call to igraph_exit_safelocale(), otherwise a memory leak will occur.

This function tries to set the locale for the current thread only on a best-effort basis. Restricting the locale change to a single thread is not supported on all platforms. In these cases, this function falls back to using the standard setlocale() function, which affects the entire process and is not safe to use from concurrent threads.

It is generally recommended to run igraph within a thread that has been permanently set to the C locale using system-specific means. This is a convenience function for situations when this is not easily possible because the programmer is not in control of the process, such as when developing plugins/extensions. Note that processes start up in the C locale by default, thus nothing needs to be done unless the locale has been changed away from the default.

Arguments: 

Returns: 

#include <igraph.h>

#include <locale.h>
#include <stdio.h>
#include <string.h>

int main(void) {
    const char *filename = "weighted.gml";
    igraph_t graph;
    igraph_safelocale_t loc;

    /* Attempt to set a locale that uses a decimal comma. Locale names
     * differ between platforms, and not all locales are available,
     * so the locale change may not be successful. */
    const char *locname = setlocale(LC_ALL, "de_DE");
    struct lconv *lc = localeconv();
    if (strcmp(lc->decimal_point, ",")) {
        /* If decimal point is not a comma, presumably because the requested
         * locale was not available, report locale information. */
        fprintf(stderr, "setlocale() returned '%s', decimal point is '%s'\n",
                locname ? locname : "NULL",
                lc->decimal_point);
    }

    FILE *file = fopen(filename, "r");
    if (! file) {
        fprintf(stderr, "Cannot open %s file.\n", filename);
        exit(1);
    }

    /* An attribute table is needed to read graph attributes. */
    igraph_set_attribute_table(&igraph_cattribute_table);

    /* At this point, the current locale may use decimal commas.
     * We temporarily set a C locale using enter_safelocale() to
     * allow the GML reader and writer to work correctly.*/
    igraph_enter_safelocale(&loc);
    if (igraph_read_graph_gml(&graph, file) != IGRAPH_SUCCESS) {
        fprintf(stderr, "Reading %s failed.\n", filename);
        abort();
    }
    igraph_write_graph_gml(&graph, stdout, IGRAPH_WRITE_GML_DEFAULT_SW, NULL, "");
    igraph_exit_safelocale(&loc);

    igraph_destroy(&graph);

    return 0;
}

void igraph_exit_safelocale(igraph_safelocale_t *loc);

Restores a locale saved by igraph_enter_safelocale() and deallocates all associated data. This function must be paired with a call to igraph_enter_safelocale().

Arguments: